Doctrine of Jus Cogens :
A treaty may be declared void if it conflicts with the general principles of International Law. Article 53 of the Vienna Convention on the Law of Treaties 1969 lays down that a treaty is void if, at the time of its formation, it conflicts with a peremptory norm of general International Law, which is a norm accepted and recognized by the international community of states as a whole, as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general International Law having the same character. On an objection raised to the claim of invalidity of the treaty by any other party, the parties will have to seek a solution by negotiation, inquiry, mediation, conciliation, arbitration, judicial Settlement, resort to regional agencies or arrangements or other peaceful means of their choice as enshrined in Article 33 of the United Nations Charter.
In simple words :
There are Certain Principles of International Law which all the States must observe. Their non-observance may affect the very foundation of the Legal system to which they belong. They, therefore, cannot be altered by concluding treaties. These rules possess the rule 'of Jus Cogens. If a treaty is concluded which is violative of these principles shall be regarded as invalid.
1) Functions of Treaty
2) Classification / Kinds of Treaties
3) Termination of Treaties
4) Termination of Treaties by Mutual Consent
5) Formation of treaties (Various Stages in the Formation of treaties)